Edge System

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SOUTHERN METHODIST UNIVERSITY

EGPRS (EDGE) - Enhancing the GSM GPRS System

Deana Clover

Student ID: 20462108

EETS8316 Section 401, Fall 2003


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Abstract

Mobile users continue to demand higher data rates. With the continued growth in cellular

services, laptop computer use and the Internet, wireless network providers are beginning

to pay an increasing amount of attention to packet data networks. Enhanced Global

Packet Radio Service (EGPRS) offers a substantial improvement in performance and

capacity over existing GPRS services, in return for a relatively minimal additional

investment. EGPRS, commonly called EDGE, achieves these enhancements to the GPRS

system primarily by implementing changes to the Physical layer and to the Medium

Access Control/Radio Link Control (MAC/RLC) layer. The significant improvements

are a new modulation technique, additional modulation coding schemes, a combined Link

Adaptation and Incremental Redundancy technique, re-segmentation of erroneously

received packets, and a larger transmission window size.

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Table of Contents

1. INTRODUCTION..................................................................................................... 1

1.1 GPRS/EDGENETWORKARCHITECTURE ............................................................ 2

1.1.1 Mobile Stations ........................................................................................... 2

1.1.2 Base Station Subsystem (BSS)..................................................................... 3

1.1.3 GPRS Support Nodes.................................................................................. 4

1.1.3.1 Serving GPRS Support Node (SGSN).................................................... 4

1.1.3.2 Gateway GPRS Support Node (GGSN) ................................................. 4

1.2 GPRSSESSION OVERVIEW .................................................................................. 5

1.2.1 GPRS Attach ............................................................................................... 5

1.2.2 Packet Data Protocol (PDP) Context Activation ....................................... 6

1.2.3 Data Transfer.............................................................................................. 6

2. PHYSICAL LAYER................................................................................................. 7

2.1 CHANNEL CODING,INTERLEAVING AND PUNCTURING ......................................... 8

2.2 MODULATION ...................................................................................................... 9

2.3 LINKADAPTATION AND INCREMENTAL REDUNDANCY...................................... 10

3. RLC/MAC ............................................................................................................... 12

3.1 MEDIUM ACCESS CONTROL (MAC) .................................................................. 12

3.1.1 Dynamic Allocation .................................................................................. 13

3.1.2 Extended Dynamic Allocation................................................................... 13

3.1.3 Fixed Allocation........................................................................................ 13

3.2 RADIO LINKCONTROL (RLC)............................................................................ 14

3.2.1 Unacknowledged Operation ..................................................................... 14

3.2.2 Acknowledged Operation.......................................................................... 14

4. CONCLUSION ....................................................................................................... 15

4.1 PHYSICAL LAYER............................................................................................... 16

4.2 RLC/MACLAYER............................................................................................. 16

5. BIBLIOGRAPHY................................................................................................... 17

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1. Introduction

The success of cellular services combined with the increased presence of laptop

computers and the rapid growth in the Internet indicates an optimistic future for wireless

data services. However, today the population of data subscribers is small when compared

to that of voice. The primary obstacle to user acceptance appears to be the performance

limitations of the existing services and products as well as the pricing structures

associated with them.

The acronym EDGE represents Enhanced Data Rates for Global Evolution but has

become synonymously used for Enhanced Global Packet Radio Service (EGPRS) as well.

Since EDGE is the more common term in use today it will be used here over the more

formally correct EGPRS. EDGE improves the throughput rate of GPRS by enhancing the

radio transmission interface. Higher data rates are achieved by using a different

modulation scheme when channel conditions allow, and by using a link adaptation

technique known as incremental redundancy. The objective of the EDGE design was to

minimize the impact on existing GSM networks. The major modifications affect the

physical layer and the Radio Link Control and Medium Access Control (RLC/MAC)

layer so this is the focus of this paper.

EDGE is part of the evolution from 2nd

Generation networks to 3rd

Generation networks

and is often referred to as a 2.5G system. The maximum speed per timeslot for GPRS is

21.4 kbits/s while EDGE provides almost three times this speed with a maximum of 59.2

kbits/s per timeslot. A maximum of eight time slots can be employed during one data

connection to provide a theoretical maximum speed of 160 kbits/s for GPRS and 473.6

kbits/s for EDGE.

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1.1 GPRS/EDGE Network Archi tecture

Figure 2 illustrates the data transmission path of GPRS/EDGE. The GPRS Public Land

Mobile Network (PLMN) is composed of network elements and the communications

links connecting these elements. The network elements relevant to this discussion are the

Mobile Station (MS), the Base Transceiver Station (BTS), the Base Station Controller

(BSC), the Serving GPRS Support Node (SGSN), and the Gateway GPRS Support Node

(SGSN).

Gn

SGSN

Figure 2. Structure of GSM/GPRS Network

1.1.1 Mobile Stations

GSM mobile stations must be designed with the appropriate protocol layers for them to

support GPRS or EDGE. They also must be modified to operate on shared traffic

channels and the coding schemes must be added. If the MS is EDGE-capable this means

GGSN

BSC

8-PSK covera

BTS

BTS

GbA-bis

A-bis

e

GMSK covera e

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it also must implement a new modulation scheme (8-PSK). There are three classes of

Mobile Stations:

Class A: Allows for simultaneous use of GPRS/EDGE and other GSM services

(such as voice).

Class B: Alternate use of GPRS/EDGE or GSM services is possible. Only one

can be used at a time but it is possible to toggle back and forth.

Class C: Designed for GPRS/EDGE only. This class provides no voice service.

1.1.2 Base Station Subsystem (BSS)

The BSS is composed of the Base Station Transceiver (BTS) and the Base Station

Controller (BSC). The BTS is comprised of all the radio transmission and reception

equipment. It provides coverage to a particular geographic area and is controlled by the

BSC. The BSC handles the medium access and radio resource scheduling, as well as data

transmission toward the mobile station over the A-bis interface. The increased bit rate

provided by EDGE also increases the demand on the rest of the network path.

Transmission on the A-bis interface varies greatly depending on the call type in use.

Instead of allocating fixed transmission capacity according to the highest possible data

rate for each traffic channel it is much more efficient and economically practical to share

common transmission resources between several traffic channels. This common resource

is call the EGPRS Dynamic A-bis Pool (EDAP). The EDAP functionality allocates

capacity to cells only when it is needed so reserving a full, fixed transmission link per

radio does not waste resources. The size and number of EDAPs in a BSC has an impact

on the Packet Control Unit (PCU) dimensioning. The PCU is limited in the number of A-

bis channels it can support and The BSC has a limitation on the number of PCUs it can

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support. The number of PCUs selected for use also has an impact on the Gb interface and

the SGSN dimensioning.

1.1.3 GPRS Support NodesThe advent of GPRS brought two new elements to the GSM system. These are the

Serving GPRS Support Node (SGSN) and the Gateway GPRS Support Node (GGSN).

The SGSN controls GPRS service in a particular geographical coverage area. The GGSN

serves as the gateway between the GPRS network and other packet networks.

1.1.3.1 Serving GPRS Support Node (SGSN)

The SGSN handles mobility functions and controls the data flow toward the BSC over

the Gb interface. The SGSN provides a point of attachment for the GPRS mobiles. After

the mobile station has attached to the system a logical link is established between the

mobile station and the SGSN, via the base station. The SGSN is responsible for the

transport and delivery of packets to and from the user. This requires the SGSN to keep

track of the current location of each mobile station attached to it. It is responsible for

validating the mobile stations, before they are allowed access to the GPRS system, and

also performing security functions such as authentication and ciphering.

1.1.3.2 Gateway GPRS Support Node (GGSN)

The GGSN provides connectivity to the external packet data networks (PDN). The

primary role of the GGSN is to route data to the mobile stations at their current points of

attachment. All packets between the external PDNs and the GPRS network enter and exit

from the GGSN. Once the mobile station activates its packet data address, the mobile

station is registered with the corresponding GGSN. The GGSN maintains a routing table

associating the active GPRS mobiles in the system with a particular SGSN.

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The enhancement of GPRS to EDGE does not greatly impact these important network

support elements (SGSN and GGSN) with the exception of the increased demand

associated with the faster data rates that EDGE allows. The primary modifications

required are at the physical layer and the data link layer, therefore, these required major

modifications are the focus of this paper. However, before concentrating on the

differences required for EDGE it may be useful to briefly discuss the process that occurs

when a mobile user wishes to use the GPRS packet data system.

1.2 GPRS Session Overview

A GPRS session begins when a GPRS station informs the network of its presence and its

desire to be available for packet data service. The Base Station Subsystem (BSS)

coordinates this request and notifies the mobile station which resources it can use to send

a message.

1.2.1 GPRS Attach

The mobile sends a GPRS attach message to the SGSN, which triggers the SGSN to

perform authorization, to check authentication and to notify the Home Location Register

(HLR) that the user is located in this SGSN service area. The HLR provides service

profile information to the SGSN so it can coordinate the service request. The SGSN then

sends a GPRS Attach Accept message to the mobile station. To complete the attach

process, the mobile acknowledges receipt of the Attach Accept message and also of its

new temporary identity (TLLI). The mobile station still must activate its Packet Data

Profile (PDP) before it can exchange any data.

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1.2.2 Packet Data Protocol (PDP) Context Activation

To begin the PDP context activation process the GPRS station must once again request

radio resources from the BSS. When this request is granted the mobile station sends an

Activate PDP Context Request message to the SGSN. The SGSN determines if the

requested service is allowed based on the service profile information received from the

HLR. It also determines which GGSN needs to be contacted to provide the service that

was requested in the PDP Context Request. The SGSN then forwards the request to the

appropriate GGSN. The GGSN negotiates with external networks to set up the requested

service and responds to the SGSN with the Create PDP Context Response message. This

message contains the PDP address for the mobile and any additional information that

may be necessary to complete the service transaction. The SGSN stores the relevant

information and notifies the BSS of any specifics regarding subsequent traffic related to

this PDP Context. Finally the SGSN forwards an Activate PDP Context Accept message

to the mobile station, which contains the specifics of the packet session. The mobile

station can now begin its data session.

It is important to recognize the difference between a mobile station attaching to a SGSN

and a mobile station activating a PDP address. A single mobile station attaches to only

one SGSN but it may have multiple PDP addresses active simultaneously. Each of these

PDP addresses may be anchored at different GGSNs.

1.2.3 Data TransferWhen the PDP Context Activation has been completed, the data session may begin.

Communication between the SGSN and the GGSN is achieved through the use of

tunneling. This is the process of adding a header to the existing packet so that it can be

routed through the backbone network. When the packet reaches the far side of the GPRS

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network the additional header is discarded and the packet continues on its route based on

the original header. The use of tunneling helps solve the problem of mobility for the

packet networks and eliminates the complex task of protocol interworking [10]. The

GPRS system employs tunneling when sending packets from the mobile station to fixed

nodes and also when sending from fixed nodes to mobile stations. This is a distinction

from mobile IP which only uses tunneling in the second case.

2. Physical Layer

Both GPRS and EDGE adapt to the current channel conditions. During good channel

conditions they utilize coding schemes that result in the highest throughput rate possible.

During poor channel conditions they increase error protection to improve the Bit Error

Rate (BER) and thereby reduce the need for retransmissions. EDGE has the capability of

not only changing the channel coding rate but also changing the modulation technique.

GPRS uses a rate convolutional coder and then employs different amounts of

puncturing (removal of bits) to yield a code rate that is appropriate for the channel

characteristics. The different puncturing levels result in four different effective coding

rates and data rates. EDGE uses a rate convolutional coder and selects a puncturing

rate that will maximize the net throughput. EDGE has nine different modulation coding

schemes. MCS1 through MCS4 use GMSK modulation while MCS5 thorough MCS9

use 8-PSK. Incremental redundancy, also known as hybrid automatic repeat request

(ARQ) type II, [6] is achieved by puncturing a different set of bits each time a block is

retransmitted thus gradually decreasing the effective code rate for every new transmission

of the block.

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2.1 Channel coding, Interleaving and Puncturing

Channel coding is the process of adding redundancy to a data stream to render it more

resilient to impaired transmission situations. This redundancy is achieved by adding

extra bits that are used to detect and, in some cases, correct errors. The result of this

channel coding is an improvement in the Bit Error Rate (BER) but a reduction in

throughput. However, due to the increased robustness of the data stream less

retransmission should be required which translates into a final result of improved

throughput. Figure 1, shown below, displays the data rates possible with each coding

scheme available in GPRS and EDGE. Puncturing or purposely removing bits achieves

these different effective coding rates.

9.0

5

13.4

15.6

21.4

8.8

11.2

14.8

17.6

22.4

29.6

44.8

54.4

59.2

0

10

20

30

40

50

60

70

80

CS1

CS2

CS3

CS4

MCS1

MCS2

MCS3

MCS4

MCS5

MCS6

MCS7

MCS8

MCS9

Schemes

Rawd

atara

te

kbpspertime

slot

GPRS EDGE

GMSK Modulation

8PSK Modulation

Rawd

atarate

kbit/spertimeslot

Figure 1. Raw data rates achievable with EGPRS coding schemes

Radio channels are inherently susceptible to fading conditions that can introduce bursty

errors into the data transmission. Therefore the coded bits are interleaved in an attempt to

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randomize any such errors at the receiver. The process of interleaving results in output

that displays isolated errors as opposed to error clusters. This results in an increased

frequency of successful bit stream decoding. A 20ms EDGE radio block consists of one

RLC/MAC header and either one or two RLC data blocks. In order to support the

incremental redundancy feature the header is coded and punctured independently from

the data. In GPRS a radio block is interleaved and transmitted over four bursts; each one

must be received correctly in order to decode the entire radio block or it must be

retransmitted. EDGE handles the higher, less redundant coding schemes differently than

GPRS does in an effort to overcome this problem. MCS7, MCS8 and MCS9 actually

transmit two radio blocks over the four bursts and the interleaving occurs over two bursts

instead of four. This reduces the number of bursts that must be retransmitted should

errors occur [3].

2.2 Modulation

The modulation scheme employed in GPRS is Gaussian Minimum Shift Keying (GMSK)

which provides one bit per symbol. In order to increase the bit rate per time slot 8-Phase

Shift Keying (8-PSK) modulation in addition to GMSK was selected for the EDGE

standardization. 8-PSK modulation transmits three consecutive bits with each symbol.

So EDGE and GPRS both have the same symbol rate but the bit rate is higher in EDGE.

This is the primary reason why EDGE can achieve approximately triple the throughput

speed of GPRS.

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111

011010

000

100

110001

101

Q

I

Figure 2. 8-PSK constellation diagram [6]

2.3 Link Adaptation and Incremental Redundancy

The addition of incremental redundancy combined with link adaptation significantly

improves performance compared to that resulting from pure link adaptation. The radio

link quality is measured in the downlink by the mobile station and in the uplink by the

base station. This information is used to determine the most appropriate coding scheme

for the current prevailing radio channel conditions. The modulation coding scheme can

be changed for each radio block but the practical adaptation rate is usually dependent

upon the measurement interval. EDGE also adds incremental redundancy to the radio

link quality. The initial transmission of the data block may include little redundancy. If

it is not received correctly more redundant information will be sent in the next

retransmission by sending the same data block but using a different puncturing scheme.

The blocks of data containing data errors are not discarded but are stored and combined

with each new retransmission until the data block is successfully decoded. This process

results in a lower effective code rate. Thus, the maximum achievable throughput per time

slot depends on the radio channel conditions and cannot be achieved in all environments

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[6]. Three block sizes are defined for the nine modulation and coding schemes. This is

done to facilitate the retransmission process. For the retransmission of data the same

MCS or another MCS from the same family of MCSs can be selected. The three RLC

block sizes and their corresponding MCSs are shown in Figure 3 below. An example

scenario follows:

MCS9 carries two RLC blocks each 74 bytes in size. If the signal to

interference ratio gets too low or the noise gets too high a transmission errormay occur and a retransmission will be requested. The 74 bytes blocks may

then be retransmitted using MCS6 with one block per four GSM physical layer

bursts. If additional coding is required this can be further segmented into two 37bytes sub-blocks, and each can be transmitted using MCS3. The header would

indicate that this is a segmented portion of a 74 byte RLC block and not aretransmission using 37 byte blocks. Thus, EDGE provides plenty of flexibility

for block-by-block rate adaptation [7].

MCS-3

Family A 37 octets 37 octets 37 octets 37 octets

MCS-2

Family B 28 octets 28 octets 28 octets 28 octets

MCS-1

Family C 22 octets 22 octets

MCS-4

MCS-6

MCS-9

MCS-5

MCS-7

Figure 3. Relationship of the threeRLC block sizes to the EGPRS modulation cod ingschemes [7].

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3. RLC/MAC

The responsibilities of the Radio Link Control (RLC) include segmentation and

reassembly of Logical Link Control (LLC) Packet Data Units (PDU). The Medium

Access Control (MAC) has responsibility for resource scheduling and allocation. This

combination of functions determines the user performance at a system level. The

efficiency of physical layer channel utilization can be determined by the resulting

throughput and delay. The RLC/MAC header contains sequence numbers used to

identify the order of the blocks. It also contains the Temporary Flow Identifier (TFI)

that identifies the Temporary Block Flow (TBF) used to carry the data to a particular

mobile station.

3.1 Medium Access Control (MAC)

The MAC layer provides the capability for multiple mobile stations to share the same

transmission medium through the use of contention resolution and scheduling procedures.

A reservation protocol based on the Slotted Aloha protocol is used for contention

resolution among several mobile stations. The MAC layer aids in queuing and

scheduling of the access attempts. Contention can also occur within a single mobile

station when different services are competing for the same limited radio resource. The

MAC layer prioritizes the data to be sent with signaling data receiving a higher priority

than user data. The MAC layer uses three modes to control the transfer of data in the

uplink. The initial mode is specified when the Temporary Block Flow (TBF) is

established.

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3.1.1 Dynamic Allocation

Dynamic allocation allows unused channels to be allocated as Packet Data Channels

(PDCHs) and if a higher priority application requires resources the PDCHs can be

released. The mobile station monitors the downlink to determine when to send data on

the uplink. The Uplink State Flag (USF) is assigned to the mobile station during the

establishment of a TBF. The USF is included in the header of each RLC/MAC data

block sent on the downlink. It designates which mobile is allowed to transmit data in that

particular PDCH of the next uplink radio block. When the mobile station detects its

assigned USF it can transmit either a single RLC/MAC block or a set of four RLC/MAC

blocks. Because all the mobile stations constantly monitor the USF, the allocation

scheme can be altered dynamically. There are eight possible USF values, allowing up to

eight users to be multiplexed onto one PDCH.

3.1.2 Extended Dynamic Allocation

Extended dynamic allocation allowsthe mobile station to be allocated multiple time slots

in a radio block without having to monitor the USF value for each time slot. It differs

from dynamic allocation in that when a mobile station sees its USF value in a particular

downlink timeslot it assumes that it can use that time slot and all higher numbered time

slots in the allocated set during the next uplink radio block.

3.1.3 Fixed Allocation

Fixed allocation assigns the mobile station exclusive use of certain channels. The

network commands the mobile station to use fixed allocation via the Packet Uplink

Assignment message. This message also contains a bitmap indicating the specific

PDCHs, which may be used to transfer data.

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3.2 Radio Link Control (RLC)

The RLC layer is responsible for error correction, retransmission, segmentation and

reassembly. It is important to correct radio link errors before they are passed up to higher

layers. If they are passed to the Internet they will only have the opportunity to be

corrected by Transmission Control Protocol (TCP) using end-to-end transmission. This

would obviously take a long time and use a large number of resources in order to

complete the original transmission and then to complete an end-to-end retransmission.

The RLC layer uses selective retransmission to correct errors. This scheme only requires

that erroneous frames be retransmitted. The correctly received frames are buffered until

the erroneous frame is received correctly and then all the frames are placed in proper

order and sent to the upper layer, which is the Logical Link Control (LLC) layer. The

RLC layer is responsible for segmentation of Logic Link Control (LLC) layer frames into

RLC blocks suitable for transmission and also for reassembly at the destination location.

Block Sequence Numbers (BSNs) are assigned in order to complete this reassembly task

as well as to detect missing radio blocks. The RLC layer supports two modes of

operation.

3.2.1 Unacknowledged Operation

Unacknowledged operation does not guarantee the arrival of the transmitted RLC blocks

and there is constant delay. The receiver attempts to preserve the length of the data

blocks it receives. This is useful for real time applications such as video.

3.2.2 Acknowledged Operation

Acknowledged operation does guarantee the arrival of the transmitted RLC blocks.

Selective retransmission is used to retransmit data blocks that did not arrive error free.

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BSNs are used to determine which blocks are missing and to request retransmission of all

missing or improperly received blocks. The two types of retransmission schemes are

Type I ARQ and Type II hybrid ARQ. Type I ARQ is used by the receiver and the

transmitter to ensure that all bocks are delivered error free. Type II hybrid ARQ is the

more elaborate method that involves storing incorrectly received blocks and then

combining them with the retransmitted blocks in order to restore the correct original data.

For each RLC peer-to-peer entity there is a transmit and receive window size established

that allows a limited number of blocks to be transmitted prior to receiving an

acknowledgement. The window size for EDGE is set according to the number of time

slots allocated in the direction of the TBF and ranges from 64 to 192 for single time slot

operation or 64 to 1024 for 8-time slot operation. In GPRS the window size is set at 64.

The larger window size in EDGE allows more blocks to be transmitted before the

acknowledgement is required and reduces the probability of stalling the transmission

window. It also makes it possible for EDGE to use a higher operating Block Error Rate

because of the use of incremental redundancy. For this purpose a larger window is

needed to enable multiple copies of each data block without causing the window to stall.

4. Conclusion

This paper has presented an overview of EDGE with particular focus on the physical

layer and the data link layer. The goal of EDGE is to provide a packet data network that

provides operating rates that are of adequate speed for most applications. EDGE

achieves this increase in throughput rate mainly through enhancements to the physical

layer and the RLC/MAC layer of the GPRS system.

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4.1 Physical Layer

The physical layer is enhanced by the addition of 8-PSK modulation, new coding

schemes, and incremental redundancy. 8-PSK increases the bit rate by mapping three

bits to each symbol which has the effect of almost tripling the bit rate. The number of

coding schemes has been increased from four to nine permitting the selection of the

optimal rate for the current channel conditions through the link adaptation mechanism.

Incremental redundancy is the mechanism by which erroneous data packets get combined

to re-create an error free data packet.

4.2 RLC/MAC Layer

EDGE introduces re-segmentation of RLC blocks. Blocks determined to contain errors

can be retransmitted utilizing a more robust coding scheme until they are correctly

received. A larger window size is provided in EDGE that prevents the stalling of

transmission, which in turn reduces the wasteful transmission of blocks due to the RLC

protocol. The use of the combined Link Adaptation and Incremental Redundancy

scheme results in an increase in system capacity due to the reduced need for re-

transmissions. Upgrading a network to EDGE requires relatively minor changes and

results in a rather significant gain in performance and capacity.

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5. Bibliography

[1] J. Chuang, L. Cimini Jr., G. Ye Li, B. McNair, N. Sollenberger, H. Zhoa, L. Lin,and M. Suzuki, High-Speed Wireless Data Access Based on Combining EDGE

with Wideband OFDM,IEEE Communications Magazine, Nov 1999.

[2] J. Chuang and N. Sollenberger, Beyond 3G: Wideband Wireless Data AccessBased on OFDM and Dynamic Packet Assignment,IEEE CommunicationsMagazine, Jul 2000.

[3] Ericsson AB, EDGE Introduction of high-speed data in GSM/GPRS networks,White Paper AE/LZT 123 7058 R2, http://www.ericsson.com/products/white_papers_pdf/edge_wp_technical.pdf (last visited Oct 2003).

[4] A. Furuskar, S. Mazur, F Muller, and H Olofsson, EDGE: Enhanced Data Rates

for GSM and TDMA/136 Evolution,IEEE Personal Communications, June1999.

[5] A. Gurtov, M. Passoja, O. Aalto, and M. Raitola, Multi-Layer Protocol Tracing ina GPRS Network, IEEE Vehicular Technology Conference Proceedings (Fall2002).

[6] D. Molkdar, W. Featherstone and S. Lambotharan, An overview of EGPRS: thepacket Data component of EDGE,Electronics & Communication Engineering

Journal, February 2002.

[7] S. Nanda, K Balachandran and S. Kumar, Adaptation Techniques in Wireless

Packet Data Services,IEEE Communications Magazine, January 2000.

[8] V. Sami and K. Katja, Positioning Edge in the Mobile Network Evolution,Helsinki University Of Technology, Research Seminar on TelecommunicationsBusiness II, March 2003, http://www.tml.hut.fi/Opinnot/T-109.551/2003/kalvot/

Positioning_ EDGE.doc (last visited Nov 2003).

[9] J. Seraj, Class Notes. Southern Methodist University, EETS8316 WirelessNetworks (Fall Semester 2003).

[10] Tod Switzer. EDGE and GPRS Technical Overview, Training Course. Award

Solutions, Inc., Baton Rouge (May 2003).

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